A three digit seven segment display

Hi all I just got a couple of three digit seven segment displays, I was planning to make a device that displays temperture readings on a three digit seven segment display, but I just noticed that each three digit SSD has twelve pins, the seller said there were three common grounds, and one pin that does not connect to anything. How do I controll each segment seperatly? eg. display three different numbers on each digit? Thanks

Your device will be something like this.
Write a loop that drives an anode, and sinks a cathode.
Continue for all 7 or 8 anodes.
Repeat for the next cathode.

You mean display one digit, then the other, ect, and the Arduino is so fast that you can't tell?

It's the same principle behind led dimming with pwm: you turn a (fast) light source on and off so fast it looks continuously lit to the human eye. The higher the ratio between on and off time, the brighter the light will look.

I see. Thank you very much!

Its called Persistence Of Vision. It works because you can turn them on and off many times within a second, It happens so fast that you can not tell that its flashing.